Incremental tuning process for electrical resonators based on mechanical motion

ABSTRACT

The present invention is a method for adjusting the resonant frequency of a mechanical resonator whose frequency is dependent on the overall resonator thickness. Alternating selective etching is used to remove distinct adjustment layers from a top electrode. One of the electrodes is structured with a plurality of stacked adjustment layers, each of which has distinct etching properties from any adjacent adjustment layers. Also as part of the same invention is a resonator structure in which at least one electrode has a plurality of stacked layers of a material having different etching properties from any adjacent adjustment layers, and each layer has a thickness corresponding to a calculated frequency increment in the resonant frequency of the resonator.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a divisional of U.S. patent application Ser. No. 11/146,179,filed on Jun. 6, 2005 and now U.S. Pat. No. 7,328, 497, which is itselfa divisional of U.S. patent application Ser. No. 10/192,420, filed onJul. 10, 2002 (now abandoned), which is itself a divisional of U.S.patent application Ser. No. 09/961,908, filed on Sep. 24, 2001 (nowabandoned), which is itself a divisional of U.S. patent application Ser.No. 09/431,772, filed on Nov. 01, 1999 (issued as U.S. Pat. No.6,339,276), the teachings of all of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to electrical resonators employing a mechanicaltransducer and more particularly to a method for fine tuning suchresonators following batch fabrication.

2. Description of the Related Art

The need to reduce the cost and size of electronic equipment has led toa continuing need for ever smaller filter elements. Consumer electronicssuch as cellular telephones and miniature radios place severelimitations on both the size and cost of the components containedtherein. Many such devices utilize filters that must be tuned to precisefrequencies. Hence, there has been a continuing effort to provideinexpensive, compact filter units.

One class of filter element that meets these needs is constructed frommechanical resonators such as acoustic resonators. These devices useacoustic waves, bulk longitudinal waves for example, in thin filmmaterial, typically but not exclusively piezoelectric (PZ) material. Inone simple configuration, a layer of PZ material is sandwiched betweentwo metal electrodes. The resonator may be suspended in air, supportedalong its rim, or may be placed on an acoustic mirror comprised of aplurality of alternating layers of high and low acoustic impedance (theproduct of speed and density), usually silicon dioxide and aluminumnitride. When an electric field is applied between the two electrodesvia an impressed voltage, the PZ material converts some of theelectrical energy into mechanical energy in the form of sound waves. Forcertain crystal orientations, such as having the c axis parallel to thethickness of an Aluminum Nitride film, the sound waves propagate in thesame direction as the electric field and reflect off of theelectrode/air or electrode/mirror interface.

At a certain frequency which is a function of the resonator thicknessthe forward and returning waves add constructively to produce amechanical resonance and because of the coupling between mechanicalstrain and charge produced at the surface of a piezoelectric material,the device behaves as an electronic resonator; hence, such devicescombined in known architectures can act as a filter. The fundamentalmechanical resonant frequency is that for which the half wavelength ofthe sound waves propagating in the device is equal to the totalthickness of the piezoelectric plus electrode layers. Since the velocityof sound is many orders of magnitude smaller than the velocity of light,the resulting resonator can be more compact than dielectric cavityresonators. Resonators for 50 Ohm matched applications in the GHz rangemay be constructed with physical dimensions approximately 100micrometers in diameter and few micrometers in thickness.

The resonant frequency of the resonator is a function of the acousticpath of the resonator. The acoustic path is determined by the distancesbetween the outer surfaces of the electrodes. When batch producingresonators on a substrate, the thickness of the transducing material andthe electrodes is fixed at fabrication; hence, the resultant resonancefrequency is also fixed. Since there are variations in thickness fromdevice to device resulting from manufacturing tolerances, some methodfor fine tuning the resonance frequency of each device is needed.

To compensate for this inability to reliably and inexpensively massproduce resonators with the proper resonance characteristics, it isknown to intentionally produce resonators having a lesser thickness thanthe thickness indicated to achieve a desirable resonant frequency, andthen deposit excess material on at least one of the electrodes to changethe overall thickness of the device and thereby fine tune the device. Asthis deposition of material may be done while the device is subjected toan input signal and simultaneously tested for resonance this method hasproduced acceptable results.

This method is not, however without problems as the presence of a maskneeded to control the deposition over the desired electrodes createsproblems of its own. If the mask, for instance is in contact with theelectrode, the mask mass is added to the device mass and alters theresonance characteristics of the device. On the other hand if the maskis not in contact with the device the control of the deposition areasuffers. Such masking techniques have been successful with quartz typeresonators that are much larger, but have not been as successful withresonators of the order of less than one millimeter.

It has also been proposed to remove material from the device in order toadjust its resonant frequency by etching material off the top electrodeof a resonator. With current technology, however, etching is not ascontrolled a process as deposition. Etching tends to be less uniform,smooth or reproducible than deposition. In fact prolonged etching may incases change the composition, morphology, grain nature or roughness ofthin films. Accurate etching processes require precise knowledge of therate at which material is removed to permit stopping at the exact momentthat sufficient material has been removed to produce the desiredresonant frequency. To a certain extent lack of precise control of theetching rate may be alleviated by monitoring the device frequency duringthe etching process.

When removal of material is done in a dry etching process it is usuallypossible to monitor the resonant frequency of the device during theetching process. However, monitoring of the resonant frequency duringetching is not possible when wet etching processes are used. Wetprocesses are desirable as they are much faster than dry processes.

There is thus still a need for a process to accurately fine tune amechanical resonator to a desired frequency without concern for possibleover-etching and without need to monitor the frequency during theetching process.

SUMMARY OF THE INVENTION

The above object is obtained in accordance with this invention by amethod for adjusting the resonant frequency of a mechanical resonator,the method comprising using alternating selective etching to removedistinct adjustment layers from an electrode comprising a plurality ofstacked adjustment layers, each of said adjustment layers havingdistinct etching properties from any adjacent adjustment layers.

Such a process alleviates the need to know the precise rate of etchingin a particular process because etching will stop when the etchingprocess removes all the material of one layer and reaches the next layerthat is selected to be impervious to the etching process. In other wordsthe etching stops each time at the barrier, i.e. the change from onematerial to another, as each layer is sequentially removed.

Because the stacked layers have been created by deposition of materialon the top electrode, complete removal of each layer maintains theuniformity of the remaining layer obtained during the deposition of thislayer. The composition and morphology of the unetched layer film remainsideal.

In more detail, the proposed method is a method of manufacturing amechanical resonator having a desired resonant frequency by a processcomprising:

(a) forming a first electrode;

(b) forming a transducer layer over the first electrode;

(c) forming a second electrode with a plurality of discreet layers ofknown thickness, each having etching properties different from at leastone other;

(d) sequentially etching a calculated number of the discreet layersthereby incrementally reducing the resonator overall thickness by aknown amount to adjust the resonator resonant frequency to the desiredresonant frequency.

The distinct layers are composed of materials that have differentetching properties and have thickness calculated to represent a selectedfractional increment of the resonant frequency.

More particularly the present method includes first forming the secondelectrode with an initial conductive electrode layer having a thicknesscalculated to produce a resonator having a first resonant frequency thatis higher than the desired resonant frequency. Subsequently, calculatinga desired thickness for an adjustment layer such that when theadjustment layer is placed over the first conductive layer the resonantfrequency of the resonator is reduced by a selected frequency increment.This selected frequency increment is a small fraction of the desiredfrequency correction for the resonator.

Having determined the thickness and number of adjustment layers toproduce over the conductive layer sufficient to bring the top electrodethickness to a point such that the resonant frequency of the resonatoris below the desired resonant frequency, each of these layers is createdusing materials having etching properties different from the etchingproperties of any adjacent adjustment layers. Then, the actual resonantfrequency of the resonator is measured and the number of adjustmentlayers to be removed to incrementally adjust the actual resonatorfrequency to the desired resonant frequency determined.

Once this number is known, the process according to this inventioncomprises sequentially selectively etching the calculated number ofadjustment layers to adjust the resonator resonant frequency to adesired frequency.

The terms “different etching properties” and “selective etching” as usedherein mean that the materials used may be etched using an etchingprocess for one that does not effect the other, so that one material canbe removed completely without substantially effecting the other. Thusselective etching is the process of subjecting two or more materials toan etching process that effects only one of the materials.

It is a further objective of this invention to provide a method ashereinabove described, wherein there are at least two resonatorselectrically connected and wherein the step of forming said adjustmentlayers comprises forming a first plurality of stacked alternatingadjustment layers having first and second etching properties on one ofsaid at least two resonators, and forming a second plurality of stackedalternating adjustment layers having third and fourth etchingproperties, and alternatively selectively etching said first and saidsecond pluralities of alternating stacked layers to remove saidcalculated number of adjustment layers to adjust the resonator resonantfrequency to a different desired frequency for each of said at least tworesonators.

It is also an object of the present invention to provide a mechanicalresonator comprising a first electrode, a transducer and a secondelectrode wherein the second electrode comprises a conductive layer anda plurality of distinct stacked adjustment layers, each of theadjustment layers having distinct etching properties from any adjacentadjustment layers. Preferably, the first electrode is a bottom electrodeplaced over a supporting substrate, and the second electrode is a topelectrode over the transducer.

The mechanical resonator resonant frequency is a function of theresonator thickness and the stacked adjustment layers each have athickness such that removal of an adjustment layer increases theresonant frequency by a known increment.

The adjustment layer thickness may be uniform for all layers, or maydecrease for adjustment layers closest to the conductive layer.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention can be more fully understood from the followingdescription thereof in connection with the accompanying drawingsdescribed as follows.

FIG. 1 shows a cross section of a typical resonator.

FIG. 2 shows a cross section of a resonator in which the top electrodeis structured with multiple layers in accordance with an embodiment ofthe present invention.

FIG. 3 shows the resonator according to FIG. 2 after it has beenadjusted to a desired resonant frequency according to this invention.

FIG. 4 shows a cross section of a resonator of an alternate embodimentof this invention in which the top electrode is structure with multiplelayers of different thickness.

DETAILED DESCRIPTION

Throughout the following detailed description, similar referencecharacters refer to similar elements in all figures of the drawings.Depending on the thin film materials used, additional layers ofinsulation, protective films, encapsulation, etc. may be required andall such layers and films have been omitted herein for simplificationand better understanding of the invention. The specific structure andfabrication method illustrated is for exemplary purposes only and othermethods of fabricating a resonator and or filter in accordance with thepresent invention can be devised including but not limited to substrateetching, adjustment layers, reflecting impedance matching layers, etc.U.S. Pat. No. 5,373,268, issued Dec. 13, 1994, with the title “Thin FilmResonator Having Stacked Acoustic Reflecting Impedance Matching Layersand Method”, discloses a method of fabricating thin film resonators on asubstrate.

Referring now to FIG. 1, there is shown a typical structure of amechanical resonator 10 on a support 12. The resonator structurecomprises a substrate 12 having an upper planar surface 14. Substrate 12can be any convenient material that is easily workable, e.g. any of thewell known semiconductor materials. In the present specific example,substrate 12 is a silicon wafer normally used for fabricatingsemiconductor products. Other materials useful as resonator supportsinclude, inter alia, glass, quartz, sapphire or high resistivity silicon

In the example illustrated in FIG. 1, a plurality of alternating layersof SiO2 and AlN, ending with a SiO2 uppermost layer, form an acousticreflective mirror 16. Each of the mirror layers has a typical thicknessthat is a ¼ wavelength of the filter's central frequency. For PCScellular phone applications this frequency is 1.9 gigahertz.

The use of an acoustic mirror of course, is not the only way to make aresonator. What is needed, and what the acoustic mirror provides, isgood acoustic reflection at the boundaries of the transducer layer.Other techniques to achieve this are known in the art, including using asolid to air interface. Air against most solids produces the requiredacoustic reflection. For example, one can also make an acousticresonator by thin film deposition of the resonator material on asubstrate of Si and subsequent removal of the layers beneath theresonator by: a) back etching away the Si or b)deposition of asacrificial layer beneath the resonator which is removed by subsequentpreferential etching. The present invention is directed to resonatortuning by selective etching techniques, and applies to all resonatorsregardless of their structure.

A conductive layer forming bottom electrode 18 is deposited andpatterned (if required) on the surface of the acoustic mirror. Amechanical transducer layer 20, such as a piezoelectric layer, is nextcoated over the bottom electrode, and a conductive layer 22 is coatedover the transducer layer and patterned to form the resonator 10.

In the figures used to explain the present invention the differentlayers have been shown as co-extensive layers extending only in the areaof the resonator. This is done to avoid cluttering the illustrations. Inmost applications, as is well known to the person skilled in this art,the piezoelectric layer is coated as a continuous conforming layer overthe bottom electrode, the acoustical mirror, if present, and thesupport. Similarly the acoustic layers may extend past the bottomelectrode on either side. The transducer is defined by the combinationof elements between the top and bottom electrodes in the area under thetop electrode. With the exception of the top electrode any of theseelements may be layers extending outside the top electrode covered areawith little effect on the resonator characteristics.

The top electrode may be a single conductive layer 22 as shown or acomposite of more than one preferably coextensive layers, at least oneof which is conductive, preferably the layer in contact with thetransducer layer.

The manner of fabrication of the above described layers and resonatorstructure is well known in the resonator fabrication art. The differentlayers can for example be fabricated utilizing any of the well knowntechniques, such as, vacuum deposition of a convenient material,electroless deposition, etc., followed by masking and etching to createddesired patterns.

Because piezoelectric materials are the most commonly used transducermaterials, we describe this invention using a piezoelectric material forthe transducer. Such use is not, however, intended to limit theinvention to piezoelectric transducers. Other transducers such amagnetostrictive or electrostrictive may equally well be used inresonator designs and the teachings of this invention apply equally wellto structures that incorporate different transducer materials. What issignificant is that the transducer material used results in a resonatorhaving a resonant frequency that is dependent on the overall thicknessof the resonator, which thickness includes both the transducer thicknessand the electrode thickness.

The person skilled in the art will recognize that the resonators may besubstantially more complex than illustrated, however the structure asrepresented is sufficient to explain the invention, any omitted featuressuch as details of the resonator supports, connections, protectivelayers etc. being well known in the art as previously mentioned.

FIG. 2 illustrates the first step in adjusting the resonant frequency ofa batch produced resonator according to the present invention. The batchproduced resonator will have a structure similar to the structure shownin FIG. 1. The combined thickness of the bottom electrode 18, thetransducer layer 20 and the top conductive layer 22 are calculated suchthat the resonant frequency of the resonator 10 as batch produced isabove a desired frequency, f_(d). The actual frequency is next, if sodesired, measured and a thickness of the top electrode sufficient tobring the resonant frequency to a second frequency f_(s) below thedesired frequency calculated. In the alternative, the second frequencymay be simply estimated, without measuring the actual batch producedresonator frequency, by providing a sufficient number of stacked layerto reduce the resonant frequency to well below the desired one. Next anumber of layers of material preferably co-extensive with the topelectrode are deposited on the top electrode. This can be preferablyachieved by depositing all layers and then masking once and patterningthe entirety in one etching sequence. The thickness of each of thedeposited layers is calculated to produce a known incremental change inthe resonant frequency. Thus as shown in FIG. 2 five additional layershave been deposited over the top electrode 22 bringing the resonantfrequency of the resonator below the desired resonant frequency.

As illustrated in FIG. 2 the added layers are distinct layers ofmaterials having different etching properties. Thus for example layer 18may be an aluminum layer, layer 24 a gold layer, then again layer 26 analuminum layer, layer 28 a gold layer, and again layer 30 an aluminumlayer and layer 32 a gold layer.

Because the incremental effect of each layer to the resonant frequencyof the resonator is known, one can now measure the frequency of theresonator 11 with the top electrode layers as shown in FIG. 2 and thendetermine how many layers must be removed to obtain the desiredfrequency for this resonator. Assuming that four layers have beendetermined that they must be removed, the resonator may be firstsubjected to a first etching process whereby the process only etches thegold electrode. Thus only one layer will be removed in this step asshown in FIG. 3. Next the resonator is subjected to a second etchingprocess removing the now exposed aluminum layer 30 until the layer iscompletely removed, and the next gold layer exposed. The process isrepeated as many times as needed to remove the calculated number oflayers resulting in a resonator as shown in FIG. 3 wherein the topelectrode is shown as having two layers only.

This process is particularly useful in cases where it is not possible tomonitor the shift in frequency of the resonator during etching to obtainfine tuning of batch produced resonators, as is typically the case wherewet, or chemical vapor etching is used. The ability to accurately usewet chemical etching with predictable results allows more flexibility inmaterials selection for the top electrode of resonators and highermanufacturing speeds.

In one application of this technique, resonators having resonantfrequencies that differ by a small amount may be produced in a singlebatch, and their differing resonant frequencies easily adjusted for eachby removing different numbers of layers to obtain the slight shift inresonant frequency required in certain combinations of multipleresonators.

FIG. 4 shows, in admittedly exaggerated form, an alternate embodiment ofthis invention in which the layers added to the top conductive layer 22have different thickness. Different thickness may be resorted to,depending on the material used and the ability to control the thicknessuniformity of each layer during the deposition. Thus the gold layer 24′over the aluminum layer 22 may have a first thickness that is less thanthe thickness of the next aluminum layer 26′ and so on. According tothis invention the thickness of the adjustment layers will be determinedby the desired end result, the materials and the etching processesavailable and does not have to be identical for all layers.

According to the present invention a different material is used foralternating layers of the top electrodes. In the simplest case differentmetals are used for each electrode but the desired effect of selectiveetching can also be achieved using both conductive and non conductivelayers, as well as using more than a combination of two differentmaterials.

Aluminum and gold are etched in different etchants, therefore pairingaluminum and gold for the top electrode layers allows the eventualselective etching of each electrode to obtain the necessary incrementalfrequency adjustment. The same is true for the pair Aluminum and SiO2.

Removal of excess electrode thickness is done by etching the excessmaterial from the top layer. Selective etching according to thisinvention may be accomplished using RIE with combinations of gasses thatetch the different layers selectively. For example, Chlorine basedchemistry, will not etch SiO2 as fast as Aluminum. Fluorine basedchemistry on the other hand will. One can thus use chlorine to etch thealuminum top layers and fluorine for the SiO2 sequentially until after anumber of pre-calculated cycles sufficient layers have been removed toobtain the desired resonant frequency.

Reactive ion etching or vapor phase etching are typically used becausethey would permit the simultaneous testing of the resonator while it isbeing etched. Testing for resonant frequencies may sometimes beimpractical as for instance in cases where multiple resonators are usedin an electrical circuit and access to a particular resonator may bephysically difficult. The present invention alleviates the need forcontinuous monitoring of the etching process since the processterminates automatically when all of the layer has been removed.Naturally monitoring may still be performed when using the presentinvention, and still reap the advantages of automatic termination of theetching process each time a layer is totally removed, as discussed inthe summary of the invention above.

The present invention, therefore, permits the accurate use of otheretching techniques such as wet and vapor chemical etching.

Wet etching by dipping the parts in solution offers the advantage ofspeed and can also be used to practice this invention. A subsequenttimed immersion of sufficient length removes a layer and stops. Next theresonator is dipped in a different bath and the next layer removed. Andso on until the desired number of layers are removed. The baths may beEDTA Peroxide to etch a titanium layer, and PAE etch for aluminum, incases where the layers are alternating layers of aluminum and titanium.If a gold layer is used, a potassium iodide/iodine bath can be used forthe gold layer.

Vapor phase etch is another possible process and tools exist and can beused. Similar chemistry to the wet etch example above can be used.

Etching is well known technology not requiring further discussionherein, as shown by the following two treatises: Vossen and Kern, Thinfilm processes; Academic Press, San Diego 1978 and by the same authors,Thin film processes II, Academic Press, San Diego 1991.

Single resonators are useful for single frequency applications such asoscillators or other very narrow frequency applications. In some casesthere is need to tune two resonators to two different frequencies tomake broader bandwidth filters. According to the present invention, insuch filters, one resonator is made of alternating Ti and Al stackedlayers calculated as hereinabove described, and the other is Au andSiO2. By use of combinations of nonselective removal such as Ar RIE orchemical mechanical polishing (CMP) and the selectivity of EDTA/peroxidefor Ti, PAE for Al, KI/I for Au, and chlorine based RIE for Al, one canperform the incremental tuning of this invention on each resonatorseparately without interfering with the other. Other pairs of materialsand etching process may of course be used the above been given by way ofillustration rather than limitation.

The invention has heretofore been described with reference to specificmaterials and etching processes. Such description is only for thepurpose of explaining our invention and the person skilled in the artwill recognize that there are alternate ways to practice this invention.For example, while the description of the resonator refers to a top anda bottom electrode, with the stacked layers comprising the topelectrode, it is also possible in resonator structures where the etchingprocess can be applied to either electrode that the stacked layers maybe part of either or both electrodes. Such modifications are to beconstrued as being encompassed within the scope of the present inventionas set forth in the appended claims wherein.

1. A method of manufacturing a resonator having a resonant frequency,the method comprising: (a) forming a resonator having a transducer layerbetween first and second electrode layers; (b) forming a plurality ofstacked adjustment layers over the resonator, wherein at least oneadjustment layer has an etching property different from an etchingproperty of an adjacent underlying layer; (c) measuring the resonantfrequency of the resonator with the plurality of stacked adjustmentlayers; and (d) then sequentially removing one or more of the adjustmentlayers to adjust the resonant frequency of the resonator based on themeasured resonant frequency.
 2. The method according to claim 1, whereinthe resonator is a mechanical resonator.
 3. The method according toclaim 1, wherein step (a) comprises forming the resonator on asubstrate.
 4. The method according to claim 3, wherein step (a)comprises: (a1) forming an acoustic mirror on the substrate; and (a2)forming the resonator over the acoustic mirror.
 5. The method accordingto claim 1, wherein step (d) comprises sequentially removing the one ormore adjustment layers to adjust the resonant frequency of the resonatorto achieve a desired resonant frequency.
 6. The method according toclaim 5, wherein step (b) further comprises selecting a number of layersfor the plurality of stacked adjustment layers such that the resonantfrequency of the resonator with the plurality of stacked adjustmentlayers is lower than the desired resonant frequency.
 7. The methodaccording to claim 1, wherein step (d) further comprises calculating anumber of the adjustment layers to be removed based on (1) the measuredresonant frequency of the resonator with the plurality of stackedadjustment layers and (2) resonant frequency adjustment incrementscorresponding to the adjustment layers.
 8. The method according to claim7, wherein at least two of the adjustment layers correspond to differentresonant frequency adjustment increments.
 9. The method according toclaim 7, wherein all of the adjustment layers correspond tosubstantially a single resonant frequency adjustment increment.
 10. Themethod according to claim 1, wherein step (b) further comprises: (b1)selecting at least one desired resonant frequency adjustment increment;and (b2) designing at least one of the adjustment layers to correspondto the at least one specified desired resonant frequency adjustmentincrement.
 11. The method according to claim 1, wherein step (d)comprises removing the at least one adjustment layer by etching.
 12. Themethod according to claim 11, wherein the at least one adjustment layeris removed by wet etching.
 13. The method according to claim 11, whereinthe etching removes the at least one adjustment layer and substantiallynone of the adjacent underlying layer.
 14. The method according to claim1, wherein: steps (a)-(c) are implemented to form at least first andsecond resonators on a single substrate, each resonator with acorresponding plurality of stacked adjustment layers; and step (d) isimplemented to remove different sets of adjustment layers from the firstand second resonators.
 15. The method according to claim 14, wherein thefirst and second resonators with their corresponding pluralities ofstacked adjustment layers have different measured resonant frequencies.16. The method according to claim 14, wherein different numbers ofadjustment layers are removed from the first and second resonators toachieve different desired resonant frequencies.
 17. The method accordingto claim 14, wherein: the plurality of stacked adjustment layers for thefirst resonator comprise a first pair of layer materials; and theplurality of stacked adjustment layers for the second resonator comprisea second pair of layer materials different from the first pair of layermaterials.
 18. The method according to claim 17, wherein the etchingproperties of the first pair of layer materials are different from theetching properties of the second pair of layer materials, such that theone or more adjustment layers of the first resonator are removedsubstantially independently of the removal of the one or more adjustmentlayers of the second resonator.